1. Field of the Invention
The present invention generally relates to stave coolers used with refractory for the interior linings of shaft furnaces, and in particular to thin stave cooler and support frame systems that substantially reduce manufacturing and installation costs.
2. Description of the Prior Art
Metallurgical furnaces are used for the smelting, cleaning, and converting of ore or concentrate for the production of metals. They can also be used to melt or heat metal as in the case of electric arc furnaces for the production of steel. Blast or shaft furnaces used to smelt iron or lead ore are examples of metallurgical furnaces.
Iron-making blast furnaces used in the metallurgical industry enclose the smelting processes inside a vertical cylindrical, steel containment shell. The ore and coke fuel are dropped in from the top and water-cooled nozzle tuyeres inject very hot streams of combustion air in and up from below. Liquid metal drains out the bottom through a taphole, and higher up a mixture of impurities and flux is drawn off through a slag hole.
The bosh is the hottest part of the furnace and is an inverted truncated cone that is closest to the reactions between the coke and the injected blast air. The bosh, barrel, lower stack, and other parts of a blast furnace are subjected to such severe high temperatures inside the steel containment shell would easily melt if it were not protected from the heat.
So modern blast furnaces line the inside surfaces of the steel containment shells with stave coolers made of cast-iron, or more recently of copper. These stave coolers are arrayed together like wall tiles and their cold faces are individually mounted to the inside walls of the containment shell.
Conventional cast-iron and copper stave coolers include their own mounting provisions directly inside the castings or machining of the main body. In the case of copper stave coolers, the added copper material needed to support the weight and loads, and needed to accommodate the mountings can add up to a significant, and unnecessary expense.
Horizontal grooves, pockets, and other kinds of deep textures are often provided on the hot faces of the stave coolers so that they can be lined with refractory brick, castable, or ram. An accretion layer of slag forms over the faces on top of the refractory during operation. Such an accretion layer of slag is important to promote and retain throughout the furnace's campaign life because it provides an important first layer of heat insulation.
Water cooling pipe connections are brought out through holes in the steel containment shell and all the stave coolers are interconnected together and manifolded to a cooling plant to circulate the water. The steel containment shell and piping are sealed so that process gases cannot escape through the water pipe connection holes.
Metal stave coolers are most often manufactured from cast iron or copper, and drilled and plugged copper billets. The cast iron stave coolers are typically used in the upper portion of a blast furnace where the level of abrasion by the charged material is greatest, but the heat loads are relatively low. Copper stave coolers are more expensive, but provide the greater cooling performance necessary in the hottest parts of the blast furnace.
Copper stave coolers are conventionally milled from billet, or cast in molds. The water passages in billet stave coolers are formed by either drilling a cold billet, or by hot forming the holes when the billet is extruded. Such water passages are generally round or oval in cross-section. Beginning and ending cross-connections for water circulation must be drilled into each cooling passage, one feed and one discharge.
The manufacturing of billet copper coolers can require hot working the billet, machining to achieve final outside dimensions, drilling of water passages, additional machining for plugs to close the ends of the drill holes, pre-heating for subsequent welding, welding of the various hole plugs, leak testing, and many other steps. Hot working of copper billets, e.g., by hot rolling or forging, is needed to reduce the grain size of the copper. Large copper grain sizes can result in water leaks and wall thickness defects. Smaller grain sizes improve both the leak tightness and notch toughness of the copper.
Any impurities and residual oxygen in the copper materials can cause porosity and other discontinuities when the copper stave coolers are welded or brazed. So small amounts of deoxidants are usually added to the billet material to minimize any porosity in the weld metal around the hole plugs that would otherwise result.
Billets with water passages formed during a hot extrusion process cannot be hot worked. The water passages would be distorted during the hot working.
Each feed and discharge of a billet cooler needs a connecting pipe or manifold to circulate the cooling water. Conventional connecting pipes for billet copper staves are typically welded to the cold face on the backside of the metal stave cooler. But substantial differential movements that develop between the furnace containment shell and the metal stave coolers during operational heating can cause pipe cracking and leaking of the cooling water. Any corrosion of the cooling pipe and plug weldments can also cause water leakage over time. Such inevitably leads to much shortened campaign lives for coolers built without resolving these issues.
Simply using sand cores to form the water passages can easily result in leaks. So some cast copper and low-alloy copper stave coolers cast-in an internal pipe coil to circulate the water. Without such cast-in pipe coils, leak tightness becomes a problem if the copper crystal grain sizes are too large. Keeping the grain sizes small enough is very challenging and expensive. Using a cast-in pipe coil carries its own set of problems, selecting a proper alloy material and method to use for a cast-in pipe coil are critical because there needs to be an excellent bond between the outside walls of the pipe coil and the copper cast later around it.
The conventional manufacturing of cast copper stave coolers requires many steps, including pattern making, sand filling and firing, melting of copper and addition of deoxidants, transfer and pouring of molten copper, fabrication of pipe coils, setting the pipe coils and other hardware into the mold, fettling of the solidified casting from the mold, removal of excess copper, final machining and testing.
Modern copper foundries are able to match the thermal conductivity of the cast copper to nearly equal that of billet copper used for hot working. Although the resulting grain size of the cast copper is a bit large, leakage is not a concern because an embedded pipe coil is used to circulate the water.
Given stave coolers with equal external dimensions, billet coolers which are not hot worked are the least expensive but the most likely to leak. Billet coolers which have been hot worked are somewhat more expensive and less likely to leak. Cast stave coolers with an internal pipe coil are the least likely to leak and thereby have the longest projected campaign life.
The selection of which stave cooler type to use and its dimensions are driven by commercial as well as technical reasons. Making stave coolers thinner can reduce the final cost to the customer, but thinner stave coolers become weaker. So conventional stave coolers have arrived at a balance between cost and heft.
In order to protect the water pipes as they pass through the furnace containment shell and refractory, protection sleeves, couplings, and pipes are typically installed around the water circulating pipes. These protection devices can complicate the installation of a stave cooler. Such installations can be further complicated by the need to provide adequate vertical and lateral support for the stave cooler inside the furnace containment shell. The tradeoffs invariably involve consideration for the manufacturing costs as well as the installation time, labor, and materials needed in the field.
Vertical coolers or stave coolers for metallurgical furnaces can be flat, curved or bent in either the vertical or horizontal plane, to suit the shape of the crucible. Flat stave coolers are often used in iron making blast furnaces since the diameters of these types of furnaces are so large. The stave coolers may be arranged with gaps between, fit tight to one another, or have overlap joints to prevent the passage of melt or process gases. Stave coolers are also expected to hold and support refractory insulation materials and slag accretions.